The present invention relates to a separation process for removing suspended metal hydroxides, and metal oxides from a mother liquor containing these materials. Further, dissolved impurities including phosphate ions are also removed so that a clarified effluent is obtained.
Among conventional methods for precipitating out metal ions such as Al, Fe and Zn ions from aqueous solutions thereof is floccing their corresponding metal hydroxides by neutralization thereof to promote, respectively floccing and sedimentation thereof prior to their separation from the solution.
Another known method for removing phosphate ions and the like from suspensions, such as sewage, comprises flocculating with an inorganic metal salt flocculating agent such as aluminum sulfate or ferric chloride to the form of colloidal particles which promote floc growth and improve sedimentation thereof and then separating the floc from the liquid phase.
To further enhance the floc sedimentation rate, an organic high molecular weight flocculating agent may be added to the suspension for thereby increasing floc dimension and increasing the sedimentation rate thereof. Floc developed in this manner, however, is loose, particulate, contains substantial quantities of water therein and is characterized by low density, so that although the size thereof is large, a relatively low sedimentation rate and relatively poor dehydration properties result.
In yet another prior process it has been proposed to dehydrate sludge or a highly concentrated suspension having a solids concentration of from about 10 to about 500 g/l, by adding thereto at least 0.2 g of polymer/kg of solids of a high molecular weight flocculating agent, for instance a high molecular weight polyacrylamide, a partially hydrolyzed compound thereof or sodium polyacrylate, and separating the granulated solids from the liquid phase by fluid rotation at about 0.5 - 3 m/sec., while maintaining the solids concentration at about 30 - 500 g/l. According to this method, compact agglomerate granules are obtained which facilitates water-separation therefrom. However, these agglomerate granules do not form when the untreated water has a solids concentration of less than 10 g/l; when the solids concentration is below 10 g/l, it is necessary to coagulate the sediment and condense to a solids concentration of at least 10 g/l.
Accordingly, the above-described method is inapplicable to untreated water such as sewage which contains less than 10 g/l of suspended matter and dissolved matter, said suspended and dissolved matter being metal hydroxides, metal oxides, phosphate ions, and the like.